1. Field of the Invention
The present invention generally relates to temperature sensors. More specifically, the present invention relates to a digital sensor of the internal temperature of an integrated circuit.
2. Discussion of the Related Art
An example of application of the present invention relates to the temperature characterization of integrated circuits, that is, the verification of the temperatures at which the circuit operates properly. In the final test phases of an integrated circuit, to verify or determine this correct operating temperature range, the circuit is submitted to a specific testing. This testing consists of locally creating an atmosphere heated or cooled down to the desired temperature corresponding to a test temperature. This atmosphere is projected by a pulsed air pipe as close as possible to an integrated circuit to be tested, placed on a support and put in electric relation with a test tool. Once the atmosphere at the desired temperature has been created, the operation of the integrated circuit is tested by means of an external tool. The test is repeated for different temperatures to, for example, verify the proper operation of the circuit in the provided range.
A problem which is posed in this type of application is to know the real temperature of the integrated circuit and, more specifically, temperatures of its semiconductor junctions. On the one hand, the integrated circuit is generally placed in a package which partially protects it from external temperature variations. On the other hand, in operation, the different circuit parts heat up differently, which may alter measurements.
Conventionally, an integrated sensor is used with the circuit to determine a voltage having a known variation according to temperature. Such a sensor is essentially formed of a bipolar transistor integrated in a same semiconductor wafer as the integrated circuit to be tested. The base-emitter voltage of the transistor, having a variation depending on temperature T of the semiconductor substrate, typically silicon, in which this base-emitter junction is integrated, is then measured. The current variation in the junction is imposed by the external test tool on the transistor's emitter. The variation of temperature T according to voltage can be deduced from the following formula:
      I    =                  I        sat            ·              exp        ⁡                  (                                    q              ·                              V                BE                                                    n              ·              k              ·              T                                )                      ,where
I is the current imposed by the external source on the emitter of the measurement transistor, Isat is the saturation current of the base-emitter junction of this transistor, q is the elementary charge of the electron, k is Bolzmann's constant, and n is the ideality factor of the transistor.
The disadvantage of conventional test cells is that it is necessary to know the characteristics of the transistor forming the sensor and especially its saturation current and its ideality factor. In practice, a range of ideality factors has to be set, for which the current I imposed to the transistor provides a result which is assumed to be valid.
Another disadvantage of conventional structures is that it is necessary to provide two terminals of access to the measurement transistor from the outside of the circuit. These terminals, which are only used for the testing, are useless afterwards and are prejudicial in terms of bulk.